ANALYST, SEPTEMBER 1991, VOL. 116 905 High-performance Modular Spectrophotometric Flow Cell Joiio Carlos de Andrade and Kenneth E. Collins Universidade Estadual de Campinas, lnstituto de Quimica, C.P. 6154, 13081 Campinas, Sao Paulo, Brazil Monica Ferreira lnstituto Agron6mico de Campinas, C. P. 28, 1300 I Campinas, Sao Paulo, Brazil A high-performance modular flow cell is described, which can be used in photometric or spectrophotometric detector systems for analytical and preparative scale low-pressure liquid chromatography, flow injection and related techniques. The basic design is that of an inner absorption cell unit sandwiched between two rugged supports. The novel aspects of this sandwiched cell are the wide range of interchangeable flow cell units of different dimensions that can be used, and the way in which the fluid flow occurs, essentially eliminating problems with gas bubbles and giving rapid cell clearance.The cell is compact and its versatility is enhanced by using optical fibre bundles t o transmit the light beam t o the optical path of the cell and then from there t o the detector. Keywords: Modular flow cell; spectrophotometric detector; liquid chromatography; flow injection Flow cells are critical parts of the flow-through detectors used in chromatographic systems’ and also in the detectors used in other types of systems, e . g . , for continuous-flow analysis, either segmented24 or non-segmented.5 It is the measurement of some property of the fluid as it passes through the flow cell that gives the necessary analytical signal.Optical detector flow cells are usually purchased as an integral part of any commercially available detector system o r constructed on-site by the user. Most flow cells are made in one of four basic configurations: ‘Z’,h37 ‘U’,6.*--’” ‘H’,6 or conical.6-11,12 These cells differ mainly in the way the fluid stream enters and leaves the illuminated absorption chamber. The details of the liquid flow path affect the laminar and turbulent aspects of fluid flow, which in turn determines the over-all dispersion and bubble retention characteristics of the flow cell. Many commercially available and laboratory-built flow cells have serious problems with gas bubbles and suffer from long peak clearance times. We have designed a modular flow cell in which the components can be constructed in a typical machine shop at low cost.The basic unit consists of an inner absorption cell piece sandwiched between two rugged support pieces. A series of different flow cell volumes, pathlengths and fluid flow patterns can be obtained simply by replacing one inner piece with another, somewhat in the manner of changing the rotor of a high-performance liquid chromatography (HPLC) injec- tion valve. Incorporated into the inner cell design is the concept of introduction of the flowing fluid into, and its removal from, the inner light-path piece in a symmetrical way, by means of two or more entrance and exit channels, which avoids the non-symmetrical flow pattern inherent in conventional one-jet designs. Experimental Cell Design The components of the proposed flow cell are shown in Fig.1. Details of the light-source system (a 6 V, 10 W halogen lamp coupled to a compact collimating device13 incorporating a 542 nm interference filter), the sensing system (a GaAsP Hama- matsu photodiode, Model G1126-02) and the corresponding electronic circuits are not included in Fig. 1 nor are they discussed below. However, all of the measurements were made under identical optical conditions, except for the tests with the commercial U-type cell. The central piece of the flow cell (Fig. l ) , which defines the fluid flow characteristics of the detection system, can be made of any hard material that is sufficiently inert to withstand the solvents and solutes to be used. Graphite-filled poly- (tetrafluoroethylene) (PTFE) is a good choice as it is soft enough to cut the channels conveniently and it does not allow light scatter through the cell walls, as do translucent materials, such as white PTFE and Kel-F.The flow path is of a modified Z-type, with coaxial entrance and exit tubes. This set-up permits the use of cells with a shorter light path, which may be of interest in micro-scale analytical work, such as HPLC. The inlet flow stream is directed into a circular channel around the centre hole, from which it is then directed through radial channels (three in the example shown) into the centre hole. Thus the flowing fluid essentially jets in a symmetrical way from the channels towards the centre of the light path, along the inner faces of the entrance and exit windows. In the proposed cell the entrance and exit tubes consist of PTFE tubing (A in 0.d.) inserted into the appropriate holes in the cell body.The assembled cell easily supports a hydrostatic pressure of about 405 kPa without leakage. Conventional HPLC fittings could be used for applications of even higher pressure. The three radial channel design shown in Fig. 1 can be iced by that shown in Fig. 2. 0 0 B In 1 I I c out B 1 cm C Fig. 1 Cross-section and front view of flow cell components: C, flow cell body; B, support pieces for cell and detection transducer; 0, O-rings; W, windows of glass or fused silica; G. polytetrafluoroethylene sealing gasket; and D, well for detection transducer906 ANALYST, SEPTEMBER 1991, VOL. 116 The cell support pieces can be made of stainless steel, brass or hard organic polymer material and, if desired, can be made more compact than shown in Fig.1. Results and Discussion Cell Performance As the detector response should be a function of the amount of analyte present in the cell volume and independent of the existence of other components of the analytical system, such as a chromatographic column, the performance of the proposed flow cells was tested by using a single-line flow injection (FI) manifold.5 Thus the dynamic tests were carried out by injecting, in triplicate, a desired volume (e.g., 75 pl) of KMn04 solutions, having concentrations ranging from 4.0 x 10-5 to 6.0 x 10-4 mol dm-3, into the carrier stream (water), prior to its entry into the detector cell. The transient peaks obtained in this manner give an FI-type calibration graph and simulate the chromatographic peaks, permitting the actual dynamic characteristics of the detector configuration being tested to be observed.We have extensively tested cells having centre pieces of 1.5 mm i.d. and optical pathlengths of 5 mm (volume, 9 PI) and 10 mm (volume, 18 pl). The recorded peaks (Fig. 3) show a stable baseline and an excellent precision of response (see Table l), comparable to those of high quality, but expensive, commercially available constant-volume flow cells. Results obtained with a Hellma U-type, 18 p1, 10 mm optical pathlength cell (with a Zeiss PM2A spectrophotometer operating at 542 nm) are also presented. As the tests of the flow cells were carried out using a single-line FI manifold, comparisons of performance with respect to sensitivity (analytical response) were obtained a A @ B C D Fig.2 Front view of channel configurations tested: A. Z-type flow cell; B, C and D, configurations with two, three and four channels, respectively. The inlet and outlet tube geometries and the flow cell dimensions are the same for all channel configurations (see Fig. 1) through the dispersion coefficients values, D, defined as the ratio of the absorbance for the transient FI peak ( A ) to that for the steady-state signal (Ao). As all of the variables such as carrier flow rate, reagent concentration, injection volume and F -Time Fig. 3 Flow injection transient peaks obtained with a 9 yl flow cell. Sample, KMnO,; injected volume, 75 yl; and carrier flow rate, 1.6 ml min-1.Concentrations of the injected samples: A, 4.0 x 10-5; B, 8.0 x C, 1.0 x lo-,; D, 2.0 x 10-4; E, 4.0 x 10-4; and F, 6.0 x lo-, mol dm-3. The other peaks were obtained at increased recorder chart speeds 75 I 0 3.0 6.0 9.0 12.0 Flow rate/ml min-I Fig. 4 Influence of the carrier flow rate on cell clearance times. Curves A, B, C and D correspond to the flow cells of Fig. 2. Curve E corresponds to a 10 mm optical pathlength flow cell (18 pl), with channel configuration C of Fig. 2. Curve F corresponds to a 10 mm optical pathlength U-type flow cell (18 yl). Sample, 6.0 x 10-4 mol dm-3 KMnO,; injected volume, 75 yl Table 1 Average relative standard deviation (RSD) and dispersion coefficient (D) values for various flow cells. The values were obtained using a carrier flow rate of 1.6 ml min-1 and a 6.0 x 10-4 mol dm-3 KMn04 solution.The RSD values were calculated for injection volumes of 10,25, SO, 75 and 100 PI (n = 10). As the RSD values found were almost constant over this volume range, the values shown are the average Optical pathlength/ Average Flow cell Volume/yl mm RSD (%) D value* U-type configuration (Hellma) 18 10 1 .o 0.583 Three channel, 2 type (Fig. 2, C) 18 10 1.2 0.847 Three channel, Z type (Fig. 2, C) 9 5 0.6 0.790 2-type configuration (Fig. 2, A) 9 5 0.7 0.702 * Results from 75 yl injections.ANALYST. SEPTEMBER 1991, VOL. 116 907 J H G / J F E 0 C B A - Time Fig. 5 Peak shape as a function of the carrier flow rate. Sample, 6.0 x mol dm-3 KMnO,; injected volume, 75 p1; and flow cell volume, 9 PI.Flow rates: A, 1.0; B, 1.6; C, 1.9; D, 2.2; E, 2.7; F, 3.0; G, 3.4; and H, 3.6 ml min-1. The peaks were recorded at chart speeds of 6 and 63 mm min-1 the FI manifold were kept constant for this set of data, it is inferred that values of D directly reflect the performance of the cells. These results are also shown in Table 1. It can be seen that the average relative standard deviation (RSD) for the commercial U-type cell is similar to that of the proposed three-channel cell, but the value of D is markedly lower. The use of channels in both the entrance and exit sides of the proposed flow cell significantly improves the clearance time when compared with the 2-type cell having similar cell dimensions. Centre pieces with more than two radial channels all give similar results, as shown in Fig.4; hence, there is little advantage in having more than three channels. The efficiency of solute clearance for the 9 1.11 three-channel flow cell is characterized by the peak profiles shown in Fig. 5. The preliminary tests show that when radial channels are present only on the exit side of the cell, the results are similar to those obtained when radial channels are present on both the entrance and exit sides. If physical solute dispersion from the channel volume is not of concern, it is recommended that radial channels be placed on both sides of the cell, as a matter of convenience, when mounting the cell. In any configuration gas bubble problems are virtually non-existent . Applications The proposed cell can be incorporated into a dedicated detector or coupled with most conventional photometers and spectrophotometers by means of optical fibre bundles, with- out modification of the cell compartment. It functions particularly well in flow analysis systems where rapid cell clearance is desirable.This characteristic should be useful for determinations that involve frequent sampling in addition to on-line kinetic studies. 1 2 3 4 5 6 7 8 9 10 11 12 13 References Stevenson, R. 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A Practical Guide to Design and Construction, Addison-Wesley, New York, 2nd edn., 1989, p. 145. pp. 23-86. Paper 0104932H Received November Ist, 1990 Accepted May 16th, 1991